1. Field of the Invention
The invention relates to a functional device array, and in particular to a functional device array with self-aligned electrode structures and fabrication methods thereof.
2. Description of the Related Art
A conventional functional device array includes piezoelectric sensitive devices, display devices, actuation devices, loudspeaker devices, and other optoelectronic sensor devices, which are developing towards miniaturizing while increasing effective pixel areas. Typical functional device arrays contain three components: a substrate, a spacer structure, and functional structure layers of a sensor device. An array of pixel areas is defined by the spacer structure, and the functional structure layers of a sensor device are then formed in each pixel area. Therefore, precision and efficiency for defining and fabricating the pixel area is critical for functional device arrays.
U.S. Pub. No. 2006/0039044, the entirety of which is hereby incorporated by reference, discloses a method for self-alignment of Red-Blue-Green color filters on a photodetector. A photodetector is provided with a planarized device surface. Subsequently, two dielectric layers are deposited on the photodetector defining a predetermined area. Each color of the color filters is sequentially formed on the predetermined area. After an upper layer of the dielectric layers is removed, each color filter is self-aligned with the detecting region of the photodetector.
FIGS. 1A-1D are cross sections illustrating each fabrication step for a conventional functional device array 10. Referring to FIG. 1A, a substrate 11 is provided with patterned electrodes 13 formed thereon. For example, an array of electrodes 13 are defined on the substrate 11 by using a first photomask (not shown) configured with a lithography process.
Referring to FIG. 1B, a dielectric layer 14 is formed on the substrate 11. A spacer structure is defined by a second photomask 21 configured with a lithography process, as shown in FIG. 1C. Specifically, a front exposure L step is performed to define the spacer structure, wherein the opaque area 21a of the second photomask is corresponding to the pixel area, and the transparent area 21b is corresponding to the spacer structure. Defining the spacer structure 15 by the second photomask not only requires precise alignment with the substrate 11 but also requires precise alignment with a previous first photomask. As device density increases and device dimensions shrink, misalignment errors easily occur, such that a spacer 15′ deviates from its original site resulting in inconsistent pixel areas, as shown in FIG. 1C.
Referring to FIG. 1D, functional structure layers 17 of a sensor device are formed on the pixel areas, thus completing the functional device array 10. In order to compensate for spacer deviations, dimensions of the spacer 15 must be greater than the separation distance d between the patterned electrodes, thereby generating a boundary margin s to offset misalignment. However, the boundary margin s may undesirably deteriorate aperture ratio of the functional devices. That is, the aperture ratio A/W of the actual area A of the functional layers 17 to the ideal pixel area W may be reduce due to the boundary margin s. More specifically, when the boundary margin s is greater; the aperture ratio A/W is smaller. For example, when W=10 μm, the minimum feature size is about 2 μm, and the alignment precision is about ±1 μm, the aperture ratio A/W is only about 60%.